The present invention generally relates to optical configurations for interferometers and more particularly to interferometers and methods for use with translation stages having rectilinear motions.
An important family of commercially available displacement measuring interferometers (DMI) encompasses optical configurations compatible with translation stages, particularly those used for displacement primarily along X-Y and directions of travel. Typically, such interferometers rely on polarization encoding to distinguish between reference and measurement beams as they travel along the various optical paths of the interferometer. However, the presence of polarizing elements themselves account for errors that impact on accuracy and precision. Future requirements for such interferometers, most notably those intended for use in microlithography applications, will require enhanced accuracy and precision to keep pace with the ever increasing demand for fabricating smaller scale features in microelectronic devices.
Thus the invention seeks to reduce error sources by reducing the number of polarization-dependent components from the optical geometry found in many current interferometers. Such improvements will significantly reduce the possibility for several types of polarization mixing errors, and facilitate multiple wavelength operation. It may also prove useful for linear stage applications, as well as for other types of stages.
Because of the predominant motion of two-dimensional motion of X-Y stages, the measurement beams are returned to associated interferometers by means of plane mirrors rather than retroreflectors. One of the challenges in designing plane-mirror interferometers is how to accommodate the changing pitch and yaw angles of the stage. The angle ranges are typically .+-.1 mrad, which is too large for a simple Michelson type single-pass interferometer.
The standard technique for accommodating stage-mirror pitch and yaw is to double pass the interference beam, so that the mirror angles are passively compensated. This approach has several advantages, as can be witnessed by the near universal acceptance of this interferometer design, both in the differential and high-stability embodiments (C. Zanoni, "Differential interferometer arrangements for distance and angle measurements: Principles, advantages and applications," VDI Berichte Nr. 749, p.93 (1989)). The "high stability" configuration invented by Bennett is a common type of plane mirror interferometer commercially marketed (S. J. Bennett, "A double-passed Michelson interferometer," Optics Communications, 4(6), 428-430 (1972).).
Plane mirror interferometers nonetheless have fundamental drawbacks related to the overlap of the incident and reflected measurement beams propagating to and from the plane mirror. The normal means of distinguishing between the two propagation directions is by polarization, using combinations of waveplates and cube beam splitters. Defining beam paths using polarization depends strongly on the quality of the polarization components. This is particularly true of wave plates. Affordable wave plates such as mica have a retardance tolerance of 125 mrad (.lambda./50). The residual elliptical polarization after double passing results in a 12.5% leakage of the wrong polarization state into the interferometer. Quartz wave plates are much better at 12.5 mrad (.lambda./500), but they are expensive and the residual leakage of 1.25% is still significant.
Errors in polarization encoding lead to errors in the measured displacement that are typically cyclic in nature. A common difficulty related to the wave plates is multiple passing error, which contributes several nm of cyclic nonlinear behavior. The periodicity of the error is one eighth of a wavelength in a double-pass plane mirror interferometer.
The problems with polarization encoding become even more significant when designing optics for multiple-wavelength dispersion interferometry, as has been proposed for air turbulence compensation (A. Ishida, "Two wavelength displacement-measuring interferometer using second-harmonic light to eliminate air-turbulence-induced errors," 28, 473-475 (1989)). The difficulties in manufacturing the waveplates compatible with two distinct wavelengths are compounded by the amplifying effect of the air-turbulence compensation method. Depending on the wavelengths employed, the magnitude of cyclic errors is amplified by 10X to 100X. Thus cyclic errors as large as 100 nm are possible with dispersion interferometry. An object of the invention, therefore, is to provide a means of measuring displacement of an X-Y stage without relying on polarization encoding to distinguish between the incident and reflected portions of the measurement beam.
Accordingly, it is a primary object of the present invention to provide interferometer architectures that have a reduced dependence on polarization elements.
It is another object of the present invention to provide distance-measuring interferometers for use with translation stages that undergo rectilinear motion.
It is yet another object of the present invention to provide distance measuring interferometers that are relatively insensitive to the yaw and pitch that may be present in translation stages.
Yet another object is to provide an interferometer for measuring angles or tilt.
Still another object of the present invention is to provide a distance measuring interferometer for operation at two or more wavelengths.
Other objects of the invention will in part be obvious and will in part appear hereinafter when the detailed description to follow is read in connection with the drawings.